Gas sensors which work according to the principle of thermal conductivity can be used for the determination of gases or gaseous components in fluid media. “Fluid media” are understood here to mean both liquid and gaseous media. The gas to be detected or the gaseous component to be detected, also referred to in the following as “target gas”, can be present in the medium as gas or in dissolved form, wherein the fluid medium can contain, apart from the target gas, further components or dissolved substances, such as solids, gases or liquids. Specific thermal conductivity parameters of the target gas are determined first, from which further parameters, such as for example the concentration, the pressure or the partial pressure of the target gas in the fluid medium, can be derived.
Gas sensors which work according to the principle of thermal conductivity are known, amongst others, from EP 0 501 089 A1, EP 0 429 397 A1, EP 0 433 741 or DE 4 439 715 A1.
The measurement principle is based on a combination of diffusion of a target gas through a membrane with subsequent detection of the target gas by means of a thermal conductivity measurement. The employed gas sensor comprises a membrane, which is brought into contact with the fluid medium for the measurement. This membrane separates the fluid medium from a measurement chamber in the gas sensor, in which a thermal conductivity sensor is disposed. The membrane comprises for example a polymer layer or polymer film which is permeable for the target gas and at the same time impermeable to liquid, so that only the target gas can pass through the membrane into the measurement chamber. The employed polymer has a permeability matched to the target gas.
The measurement chamber is purged with a purging gas before each measuring procedure in order to remove from the measurement chamber residues from a preceding sample of the target gas. The purging gas should have a thermal conductivity differing from the target gas and in addition be chemically inert with respect to the target gas and/or the materials of the sensor, i.e. should not react with these substances.
Following this purging procedure, the target gas can again penetrate through the membrane into the measurement chamber. At least one time-dependent thermal conductivity parameter is measured with the thermal conductivity sensor, which is related to the concentration of the target gas in the measurement chamber and therefore in the fluid medium. By taking account of the membrane permeability for the target gas, the partial pressure of the target gas can then be determined from this parameter and its concentration in the fluid medium can be determined therefrom.
In addition, the gas sensor can be calibrated, as disclosed for example in EP 0 348 243 A1.
Gas sensors which work according to the principle of thermal conductivity are used in various industrial processes and process plants as well as in the laboratory. The possible uses range from CO2 determination in beverage production to N2, H2 or CO2 determination in biological processes and bioreactors. Further target gases that can be determined with such gas sensors are, for example, sulphur dioxide (SO2) or hydrogen sulphide (H2S).
Especially for the use in industrial plants, it is important that the gas sensors deliver reliable and reproducible results of constant quality.
As has been shown, the functional capability and therefore the reliability of the gas sensors are influenced by various malfunction states, such as faults in the purging gas supply and/or the penetration of moisture into the interior of the sensor.
The malfunction states lead to faulty measurements and can even lead to failure of the gas sensor.
The problem thus arises of providing a method for the functional testing of a gas sensor which works according to the principle of thermal conductivity, in order to detect malfunction states of the gas sensor already during operation and in particular on-line.